1. Field of the Invention
The present invention relates to continuously variable V-belt transmissions, and more particularly to those suitable for use on motor vehicles.
2. Description of the Prior Art
In general, continuously variable V-belt transmissions (CVT) of the type mentioned above consist of primary and secondary pulleys each in turn consisting of movable and fixed sheaves, with a metallic belt placed around the two pulleys. The drive ratio is adjusted by shifting the movable sheaves by means of hydraulic pistons.
Because such continuously variable transmissions rely on hydraulic power, they require an oil pump and hydraulic circuitry, which render the transmission very complex in structure and cumbersome. They also exert unnecessarily strong clamping forces on the belt, resulting in reduced transmission efficiency and belt durability. Furthermore, in the event of a drop in hydraulic pressure, the clamping force on the belt becomes insufficient, making it incapable of transmitting power.
In order to overcome such problems a continuously variable V-belt transmission has been proposed recently, as described in U.S. Pat. No. 4,504,247, in which the movable sheaves of primary and secondary pulleys are provided with load cam mechanisms which exert axial forces corresponding to applied torque, the load cam mechanisms being interconnected by a linkage so that the axial forces generated by the load cams are exerted on both pulleys, and in which a drive ratio selector is provided on the movable shear of primary pulley to establish the required drive ratio.
Whereas the continuously variable transmission mentioned above is satisfactory in that it generates an axial force corresponding to the torque load imposed, and does not exert excessive clamping force on the belt, it is structurally very complex due to the load cam mechanisms on the primary and secondary pulleys being interconnected by a linkage. Also, as the reactions of axial forces generated by the drive ratio selector in the form of belt clamping force are exerted on the transmission housing, the housing must have considerable rigidity.
Meanwhile, in continuously variable V-belt transmissions, if the movable sheaves on both pulleys are shifted by equal amounts when altering the drive ratio, the displacements of the sheaves would differ from their proper displacements determined by the belt. As shown in FIG. 17, the displacement A of the movable sheave of the primary pulley and the displacement B of the movable sheave of the secondary pulley, both determined by the belt, differ from each other over the whole range of torque ratio. The difference d in their displacements is largest at a torque ratio of one, and gradually decreases towards both extremes in overdrive (O/D) and underdrive (U/D) positions. However, since the movable sheaves on both pulleys are shifted by equal displacements in the above-mentioned continuously variable transmission, their actual displacements differ from their proper displacements as determined by the belt.
In the continuously variable transmission mentioned above, said difference in displacement is absorbed by displacing the load cam mechanisms. However, continuously variable transmissions for use on motor vehicles not only transmit positive torque from the engine, but also negative torque when applying the engine brake. The load cam structure is displaced by a large amount when the the direction of transmitted torque is reversed, causing a fluctuation in torque due to the rapid change in relative rotation in the load cam mechanisms, which may in turn lead to reduction in the durability of load cam mechanisms, and to a deterioration of performance.